As disclosed in U.S. Pat. Nos. 3,888,016, 3,772,503 and 3,927,474, incorporated herein by reference, digital compasses have been provided in which the angular orientation of the compass card is determined by a primary optical pickoff which involves an encoder disk driven by the magnetic sensing element of the compass. This encoder disk is provided with digital encoder markings or a digital code that in general results in the establishing of the angular position of the encoder disk and thus the angular position of the magnetic element of the compass. The accuracy to which position can be ascertained with a digitally encoded compass is on the order of 2.degree. (e.g. .+-.1.degree.).
Extremely accurate compasses are required for some seismic prospecting applications in which it is critical to determine the direction of the seismic streamer. Moreover, the use of multiple streamers at different orientations permits location of a sonar target by triangulation, doppler shift or other techniques. Instantaneous direction readout to a high degree of accuracy is especially important in towed streamer applications for real time processing of sonar data.
It will be appreciated that the accuracy of the digital encoding is directly related to the size and spacing of the encoder marks on the encoder disk which represent the limit of the angular resolution of the compass or encoder disk. Side by side light and dark areas at the periphery of the disk in general define the limit of accuracy of the optical pickoff. While for most applications this accuracy is sufficient, in other applications measuring the angular position of the compass card to one one-hundredth of a degree is often desired.